The scotch yoke has been used for many years as a means for converting reciprocating linear movement to rotary motion and vice versa. It has found application in a variety of machines, such as motors, pumps, and compressors which utilize a piston articulated within a closed cylinder (see, e.g., U.S. Pat. Nos. 283,558; 813,736; 999,220; and 2,628,602), as well as in compactors, pumps, punch presses, robots, sewing machines, generators, and material handlers.
The essential components of a scotch yoke are a crankpin rotated about a crankshaft center at an axial offset and a shuttle having a slot therein through which the crankpin is positioned. The motion of the shuttle is constrained to a linear path by a guide, frequently, a pair of opposing parallel guide surfaces. The crankshaft and crankpin move in rotary motion and may be either the driven elements or the driving elements. The shuttle moves in rectilinear motion and likewise may be the driven element or the driving element. Thus, the scotch yoke provides a means for converting linear to rotary motion and vice versa.
The slot within the shuttle must be at least as wide as the crankpin diameter and long enough to accommodate the crankpin dimension and its travel. A pair of competing objectives in the design of scotch yokes is to eliminate friction, as well as clearance at the crankpin/slot interface. Friction results in energy loss in the conversion from linear to rotary motion or vice versa and also in wear of the scotch yoke. Clearance at the interface results in a loss of motion translation, commonly called "backlash", when converting from rotary to linear motion and vice versa (i.e., there is no translation during traversal of the clearance gap), and in brinelling, spalling and vibrations when the unrestrained driving element accelerates across the clearance gap and collides into the driven element. As has been recognized for many years, clearance and friction at the slot/crankpin interface cause energy inefficiency and excessive wear and tear.
U.S. Pat. No. 4,685,342 to Douglas C. Brackett, the inventor herein, discloses a scotch yoke type motion converter having a pair of opposing, offset bearing surfaces, one on either side of the crankpin slot in the shuttle. A corresponding pair of roller bearings is arranged on the crankpin, the bearing members being coaxially and laterally displaced from one another such that each aligns with a corresponding one of the bearing surfaces of the slot within which the crankpin is positioned. In this manner, clearance at the crankpin/slot interface can be minimized to manufacturing tolerances and friction is reduced to the rolling friction of a roller bearing.
U.S. Pat. No. 5,259,256 to Douglas C. Brackett, the inventor herein, discloses a scotch yoke type motion converter having a pair of opposing gear racks provided with mating profiles, one on each side of a crankpin slot in the shuttle. A corresponding pair of pinion sectors, each of which has a tracking profile, is arranged on a crankpin. The pinion sectors are diametrically opposed to each other such that each of the tracking profiles engages a corresponding one of the mating profiles of the gear racks. In this manner, not only can clearance at the crankpin/slot interface be minimized to manufacturing tolerances, but also the load bearing capacity of the motion converter can be increased due to increased area contact between the gear racks and pinion sectors.
Whereas different motion translation applications have varying requirements with respect to loading stresses, materials, cost, etc., it remains an objective in the art to create new scotch yoke designs having alternative structural and functional characteristics while achieving energy efficiency and wear resistance. It is therefore an object of the present invention to provide an alternative scotch yoke type motion converter which tends to simultaneously minimize clearance and friction between the crankpin and the shuttle slot while maximizing the load bearing capacity of the motion converter.